It wasn't that either. It was the courts saying that the law Congress passed didn't clearly authorize the EPA to do this, so if Congress wants it they need to say so unambiguously.
Notably this means that to change it doesn't require a constitutional amendment but only an ordinary law, which is not what is generally meant by "unconstitutional".
> The Clean Air Act exists. Congress passed it.
And then the courts interpret it and if Congress doesn't like their interpretation they can pass a new law which is more specific.
> Roberts just thinks that the law shouldn't be able to do anything controversial but there is absolutely nowhere in the Constitution that says that Congress' delegation authority is limited only to uncontroversial things.
There is absolutely nowhere in the Constitution that says that Congress even has delegation authority.
